I. Field of the Invention
The present invention relates to vehicular brake systems. More particularly, the present invention relates to a method for reducing green roughness during the break- or wear-in period. Even more particularly, the present invention concerns means and methods for coating vehicular brake component(s) with a boundary layer transfer material for reducing brake roughness or shudder during the wear-in period.
II. Description of the Relevant Art
As is known to those skilled in the art to which the present invention pertains, there are two basic kinds of vehicular brake systems. One type is the drum and shoe type brake system. In this system, a brake shoe is actuated against a brake lining on the interior of a brake drum to slow and stop the vehicle. The kinetic energy of the vehicle is transformed into heat energy by the friction which occurs at the brake linings. The brake drum, therefore, becomes heated. Thus, provision must be make to remove as much heat as possible. If the heat is not removed, the brake drum will expand and possibly enlarging so much that the shoe will not engage the brake lining on the interior of the brake drum. While this type of system has long been in use, the art has turned to the disc brake system.
In this latter system, there is a brake disc, which comprises a rotor and pads. The pads are fixed pads, comprising a friction material, which engage opposite sides of the rotor to stop the vehicle. The operation of such systems is well-known to the skilled artisan.
Regardless of the type of system, it has long been observed that there is an uneven transfer of friction material between the components during the break-in period. This increases brake torque vibration and vehicle sensitivity. Consequently, there is a sensation of brake roughness which the driver feels when braking, during the wear-in period. This phenomenon is commonly referred to as "green roughness".
While the operation of a braking system of a vehicle is well-known, the understanding of braking performance, per se, is not well understood. The criteria for selecting a braking system for any given vehicle is complex and involves a wide range of testing including mounting, durability, noise and roughness evaluations. Yet, regardless of the selected system, the roughness or shudder, which occurs at the rubbing surface between the friction material and the mating surface, such as the brake rotor, is a problem the art has sought to alleviate.
In most applications, the mating surface, e.g. the rotor is gray cast iron and the friction material e.g. the pad lining, is usually, a composite material, generally comprising a binder resin, organic and inorganic fillers and a reinforcing fiber. The composition of both the friction materials and the mating surfaces is, of course, well known in the art.
Likewise, "green roughness", as noted, is a complex concept which results in the driver of the vehicle feeling a vibration in the steering wheel or chassis during braking or a pulsation in the brake pedal. This occurs on the first few braking applications of a new vehicle up to an accumulated mileage of approximately five to ten thousand miles. This roughness can occur when the brakes are at a high temperature, e.g. 400.degree. F., which is termed "hot roughness". Also, "cold roughness" can occur if the rotor or shoe is not carefully machined to have parallel surfaces.
In either case, a major cause of brake roughness is the thickness variation between the two opposing rubbing surfaces. Generally, the larger the thickness variation, the greater the roughness. With this understanding, the art has sought means and methods to overcome this roughness.
First, it has been observed that the preparation of the rotor, per se, in a disc brake system by various machining methods can influence the roughness. Typically, the two preferred methods for finishing the rotor surface are grinding or turning. Grinding the rotor creates radially and circumferentially extending grinding grooves, which are quite uneven resulting in thickness variation. Because of the thickness variation in a ground rotor, there is an uneven friction material transfer during the break-in period which amplifies the vibratory sensation. Turning the rotor creates a spiralling or helical groove configuration with minimal thickness variation. However, grinding is a much lower-cost operation and is preferred from an economic standpoint. Burnishing the rotor can be employed to alleviate the green roughness problem. However, burnishing prior to assembly is not an acceptable manufacturing procedure.
Thus, it is to be appreciated that a major advance in the art would be provided by minimizing green roughness but still using a ground rotor. It is to this to which the present invention is directed.